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Metallurgy in Production University of Hormozgan, Mirzai, 95-96

In the Name of Allah

Metallurgy in Production

1

Chapter7-Lecture 12

First semester 95-96

Mohammad Ali Mirzai

University of Hormozgan


Chapter 7 - Part 3: Heat Treatment of Steels -

Surface Hardeninig

2

Chapter7-Lecture 12


HEAT TREATMENT

BULK SURFACE

ANNEALING

Full Annealing

Recrystallization Annealing

Stress Relief Annealing

Spheroidization Annealing

AUSTEMPERING

THERMAL THERMO-

CHEMICAL

Flame

Induction

LASER

Electron Beam

Carburizing

Nitriding

Carbo-nitriding

NORMALIZING HARDENING

&

TEMPERING

MARTEMPERING

An overview of important heat treatments

A broad classification of heat treatments possible are given below. Many more

specialized treatments or combinations of these are possible.

Chapter7-Lecture 12

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Surface Hardeninig

Chapter7-Lecture 12

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Surface Hardeninig

Chapter7-Lecture 12

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Selective Hardeninig - Flame Hardeninig

Heat is applied to the part being hardened, using an oxy-

acetylene (or similar gas) flame on the surface of the

steel being hardened and heating the surface above the upper critical

temperature before

quenching the steel

in a spray of water.

The result is a hard

surface layer

ranging from 0.050"

to 0.250" deep.

Chapter7-Lecture 12

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Selective Hardeninig - Flame Hardeninig

Chapter7-Lecture 12

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Selective Hardeninig - Induction Hardeninig

Induction hardening is a process used for the

surface hardening of steel and other alloy components.

The parts to be heat treated are

placed inside a water cooled

copper coil and then heated

above their transformation

temperature by applying an

alternating current to the coil.

Chapter7-Lecture 12

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Selective Hardeninig - Laser Hardeninig

Power lasers offer rapid, high-quality and reproducible heat

treatment methods. Laser surface hardening consists of the

rapid heating of a material's surface bylaser beam, a short

hold at the target temperature, and intensive cooling due to

the high thermal conductivity of the material.

Chapter7-Lecture 12

9


Selective Hardeninig-Electron beam Hardeninig

Electron beam hardening (EBH)

is one of the most modern surface-

hardening technologies, with some

special characteristics in

comparison to other heat treatment

technologies. Electron beam

hardening is similar to other surface

heat treatment processes in that

the material to be hardened is

heated to the austenitization

temperature, held at that

temperature for a short period, and

then quenched.

Chapter7-Lecture 12

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Diffusion Hardeninig - Carburization

Carburization is a diffusion-controlled process, so the longer

the steel is held in the carbon-rich environment the greater the

carbon penetration will be and the higher the carbon content.

The carburized section will have a carbon content high

enough that it can be hardened again through flame or

induction hardening.

The carbon can come from a solid, liquid or gaseous

source; if it comes from a solid source the process is

called pack carburizing. A heating period of a few hours

might form a high-carbon layer about one millimeter thick.

Liquid carburizing involves placing parts in a bath of a molten

carbon-containing material, often a metal cyanide; gas

carburizing involves placing the parts in a furnace maintained

with a methane-rich interior.

Chapter7-Lecture 12

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Pack Carburising

Packing work in the heat resisting steel boxes with 50

mm gap with carburising material.

Heated slowly to 850 – 9250 C, maintained for 8 hrs

according to depth needed.

Temperature

Ca

rbu

ris

ing

Tim

e

Depth of case

Chapter7-Lecture 12

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Pack Carburising

Chapter7-Lecture 12

13


• Charcoal

With Barium carbonate (10 to 15%).

Process depends on presence of CO

2C + O 2 2 CO

At surface, releases C atoms

2CO CO2 + C

C dissolved interstitially at surface of

steel.

Ba CO3 Ba O + CO2

CO2 + C 2CO

Pack Carburising

Chapter7-Lecture 12

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Diffusion Hardeninig – Liquid Carburization

Mixture of salts of Sodium Cyanide, Sodium carbonate,

Sodium/barium Chloride

Melted in pots to 870- 950 C, work immersed for 5 min to 1

hour

Then basket quenched- hard and clean surface

For shallow- 0.1 to 0.25 ,mm;

Usually for small parts

Chapter7-Lecture 12

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Diffusion Hardeninig – Gas Carburization

In batch type or continuous

furnaces.

Far widely used

Clean compact plant

Heated to 900 C for 3 to 4

hours

Hydrocarbons methane and propane partly burnt in furnace,

diluted with carrier gas to get required carbon POTENTIAL ( ie

carbon content maintained in equilibrium in the surface film- of

0.8% desirable)

Chapter7-Lecture 12

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Diffusion Hardeninig – Plasma Carburization

Plasma carburization is increasingly used to improve the

surface characteristics (such as wear, corrosion resistance,

hardness and so on).

It also provides an even

treatment of components

with complex geometry

(the plasma can

penetrate into holes and

tight gaps), making it

very flexible in terms of

component treatment.

Chapter7-Lecture 12

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Diffusion Hardeninig - Nitriding

Nitriding is a heat treating process that diffuses nitrogen

into the surface of a metal to create a case-hardened surface.

These processes are most commonly used on low-carbon,

low-alloy steels. They are also used on medium and high-

carbon steels, titanium, aluminium and molybdenum.

Nitriding heats the steel part to 482–621 °C in in the nitriding

atmosphere such as ammonia gas.

Typical applications include gears, crankshafts, camshafts,

cam followers, valve parts, extruder screws, die-casting tools,

forging dies, extrusion dies, firearm components, injectors and

plastic-mold tools.

The four main methods used are: gas nitriding, salt bath

nitriding, solid nitriding and plasma nitriding.

Chapter7-Lecture 12

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Chapter7-Lecture 12

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The time the part spends in this environment dictates the

depth of the case. The hardness is achieved by the formation

of nitrides. Nitride forming elements must be present for this

method to work; these elements include Cr, Mo, and Al.

The advantage of this process is that it causes little

distortion, so the part can be case-hardened after being

quenched, tempered and machined. No quenching is done

after nitriding.

•Obtain High Surface Hardness

•Increase Wear Resistance

•Increase Tensile Strength and Yield Point

•Improve Fatigue Life

•Improve Corrosion Resistance (Except for Stainless Steels)

Chapter7-Lecture 12

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Diffusion Hardeninig – Gas Nitriding

In gas nitriding the donor is a nitrogen rich gas, usually

ammonia (NH3). When ammonia comes into contact with the

heated work piece it dissociates into nitrogen and hydrogen.

The nitrogen then diffuses onto the surface of the material

creating a nitride layer.

The advantages :

- Precise control of chemical potential of nitrogen.

- Large batch sizes possible

- With modern computer control of the atmosphere

- Relatively low equipment cost

The disadvantages:

- Reaction kinetics heavily influenced by surface condition

- Surface activation is sometimes required to treat steels with

a high chromium content

- Ammonia as nitriding medium Chapter7-Lecture 12

21


Diffusion Hardeninig – Bath Nitriding

In salt bath nitriding the nitrogen donating medium is a

nitrogen-containing salt such as cyanide salt.

The advantages of salt nitriding is that it achieves higher

diffusion in the same period time compared to any other

method.

The advantages :

- Quick processing time - usually in the order of 4 hours

- Simple operation - heat the salt and submerge workpieces

disadvantages:

- The salts used are highly toxic

- Only one process possible with a particular salt type - since

the nitrogen potential is set by the salt, only one type of

process is possible

Chapter7-Lecture 12

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Diffusion Hardeninig – Plasma Nitriding

Also known as ion nitriding, plasma ion nitriding or glow-

discharge nitriding.

In plasma nitriding, the reactivity of the nitriding media is not

due to the temperature but to the gas ionized state (electric

fields). Such highly active gas with ionized molecules is called

plasma. The gas used for plasma nitriding is usually pure

nitrogen. There are hot plasmas typified by plasma jets used

for metal cutting, welding, cladding or spraying. There are also

cold plasmas, usually generated inside vacuum chambers, at

low pressure regimes.

Advantages:

- the close control of the nitrided microstructure

- Increase the fatigue strength

- resistance to wear

- the surface hardness of tool steels can be doubled. Chapter7-Lecture 12

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Diffusion Hardeninig – Plasma Nitriding

A plasma nitrided part is usually ready for use. It calls for no

machining, or polishing or any other post-nitriding operations.

Thus the process is user-friendly, saves energy since it works

fastest, and causes little or no distortion.

Plasma nitriding is often

coupled with physical

vapor deposition (PVD)

process and labeled

Duplex Treatment, with

enhanced benefits.

Chapter7-Lecture 12

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Diffusion Hardeninig – Solid Nitriding

The solid nitriding consists in provide the diffusion of nitrogen

into the steel using a solid materials such as cyanide salt or

solid granulated (Fe4KCN). The samples are surrounded by

this compound and confined inside a metalic (Exp. Aluminum)

bowl. This pack is introduced in a muffle oven and the

temperature is set to 560 °C.

solid nitriding is a technique not well diffused of treatment, in

comparison with the processes of gas and plasma nitriding.

Chapter7-Lecture 12

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Diffusion Hardeninig –Carbonitriding

Chapter7-Lecture 12

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Diffusion Hardeninig –Carbonitriding

Carbonitriding is similar to gas carburization with the addition

of ammonia to the carburizing atmosphere, which provides a

source of nitrogen. Nitrogen is absorbed at the surface and

diffuses into the workpiece along with carbon. Carbonitriding

(around 850 °C) is carried out at temperatures substantially

higher than plain nitriding (around 530 °C) but slightly lower

than those used for carburizing (around 950 °C) and for

shorter times. Carbonitriding tends to be more economical

than carburizing, and also reduces distortion during

quenching. The lower temperature allows oil quenching, or

even gas quenching with a protective atmosphere.

Chapter7-Lecture 12

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Diffusion Hardeninig –Nitrocarburizing

Chapter7-Lecture 12

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Diffusion Hardeninig –Nitrocarburizing

Nitrocarburizing is a shallow case variation of the nitriding

process. Advantages of the process include the ability to

harden materials which are not prehardened, the relatively

low temperature of the process which minimizes distortion,

and relative low cost in comparison to carburizing or other

case hardening processes. This process is done mainly to

provide an anti-wear resistance on surface layer and to

improve fatigue resistance.

An additional advantage is that nitrocarburizing can be applied

to the same materials as is nitriding, as well as unalloyed

materials, where good wear resistance and some improved

fatigue resistance are needed at a low cost. It is widely used

for stampings, as an alternative to hard plating or even

carbonitriding.

Chapter7-Lecture 12

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Diffusion Hardeninig –comparison

Chapter7-Lecture 12

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Diffusion Hardeninig –comparison

Chapter7-Lecture 12

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End

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Chapter7-Lecture 12

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